Tantalum is a rare metal discovered in 1802. However, the metal was not widely used until the beginning of the twentieth century when a carbon reduction process was developed to produce sufficiently pure tantalum. Since then tantalum has been applied extensively in electronic, aerospace, chemical, medical and synthetic fiber industries because it has many desirable properties.
Pure tantalum has a high melting point, good thermal conductivity, excellent chemical stability and corrosion resistance. It is not reactive to air or water; and, except for hydrofluoric acid, tantalum is resistant to corrosive attack by strong acids, including aqua regia. Further, tantalum has low surface hardness and yield strength and can be machined easily. Because of these properties, tantalum has been used to make devices, such as: heaters, reactors, pump parts, valve components and measuring devices for use in highly corrosive environments in the presence of strong acids including hydrochloric acid, nitric acid, sulfuric acid and phosphoric acid. However, because of its low hardness and yield strength, the surfaces of tantalum articles are easily scored and distorted. This leads to a undesirably short operating life.
Many methods have been developed to eliminate this problem. For example, G.B. Pat. No. 702,936 describes heat treating bored tantalum spinnerets in nitrogen or carbon monoxide to improve the surface hardness. However, this treatment also changed the surface of the spinneret leading to increased blockages while spinning. It has also been suggested that the thickness of the spinneret should be increased. However, increased thickness means longer spinning passages and reduced fiber properties. U.S. Pat. No. 4,054,468 describes a process wherein stainless steel or tantalum alloy is bonded explosively to pure tantalum to form a spinneret with better yield strength than a pure tantalum spinneret of the same thickness. These improvements failed to provide a tantalum spinneret to replace the expensive gold-platinum or gold-platinum-rhodium spinneret used in the synthetic fiber industry.
The surface properties of tantalum may be improved by coating with a film of tantalum oxide (Ta.sub.2 O.sub.5). The tantalum oxide film coating improves further the chemical stability and insulation performance of tantalum. Tantalum articles so coated are useful for high performance capacitors. The tantalum oxide film is formed by anodization of the surface of tantalum metal. However, the maximum thickness of the tantalum oxide film is limited to about 2 .mu.m, independent of anodization time. Moreover, the breakdown voltage is only about 100 to 200 volts and is unsatisfactory for applications where a higher breakdown voltage resistance is required.
In Kexue Tongbao, vol. 26, No. 5, pp. 401-405 (1981), an electrochemical process for preparing lithium tantalum oxide was described. The process provides a simple and useful process for making very thin crystal chips for use as pyroelectric detectors. In this proces, a foil of tantalum, about 2.5 .mu.m to 5 .mu.m thick, was placed in molten lithium nitrate. An anodic voltage was applied to form a polycrystalline lithium tantalum oxide chip, about 7 .mu.m to 14.5 .mu.m thick, wherein the crystals have a preferred orientation.
None of these methods provide tantalum articles with sufficiently improve wear resistance and surface electrical properties.
It is, therefore, an objective of the present invention to provide tantalum articles with improved surface performance, including hardness, yield strength, elasticity, high voltage breakdown resistance and other desired physical properties.
Since the same problems exist with niobium and tantalum-niobium alloy articles, it is also an objective of the present invention to provide a method to improve the surface performance of niobium and niobium-tantalum alloy articles. A further objective is to provide a process by which the tantalum, niobium and tantalum-niobium alloy articles with improved surface properties can be made.